A TN (twisted nematic) mode has widely been used as one of display systems for a liquid crystal display. However, the TN mode has been replaced by a lateral electric field system in recent years that applies a voltage between a pixel electrode and an opposed electrode to generate an electric field in a direction substantially horizontal to a panel, thereby driving liquid crystal molecules in a horizontal direction.
The lateral electric field system works advantageously for wider viewing angles, higher resolution and higher brightness, and is considered to become the mainstream of medium and small sized panels represented by smartphones and tablets, for example.
The lateral electric field system includes known modes such as an IPS (in-plane switching) mode and an FFS (fringe field switching) mode. In the FFS mode, a lower electrode and an upper electrode with a slit are arranged while an insulating film is placed therebetween. One of the upper and lower electrodes functions as a pixel electrode and the other electrode functions as an opposed electrode. An electric field is generated in a direction from the slit of the upper electrode toward a liquid crystal above the upper electrode. The electric field thereby generated drives the liquid crystal.
In a display region of an active-matrix liquid crystal display, a thin-film transistor is formed below a lower electrode while a protective insulating film is provided between the lower electrode and the thin-film transistor. Regarding application of a voltage, an arbitrary control signal (voltage) is applied to the thin-film transistor from outside via a signal line to turn on the thin-film transistor. In response, a predetermined voltage is applied to the lower or upper electrode via a contract hole formed in the protective insulating film. Japanese Patent No. 4487318 describes an example of such a liquid crystal display panel.
In the aforementioned structure, if a conductive layer formed inside the contact hole in the protective insulating film and an electrode of the thin-film transistor are not connected stably, an electric field is not generated normally between the lower and upper electrodes, leading to display failure in some cases.
Hence, the lower and upper electrode are each composed of a transparent conductive film of a thickness of 100 nm or less in order to increase the transparency of the transparent conductive film.
This forms an interconnect inside the contact hole composed of the thin transparent conductive film, and this transparent conductive film is further formed on an electrode surface of the thin-film transistor at the bottom of the contact hole.
The contact hole is small in width so the coating performance of the transparent conductive film is reduced at the inner side wall of the contact hole. Hence, with the intention of preventing cut of the transparent conductive film at a stepped part (step cut), extreme care should be taken so as to maintain a sufficient thickness of the transparent conductive film on a part hard to cover with the transparent conductive film.
In the liquid crystal display panel disclosed in Japanese Patent No. 4487318, a contact hole to be formed in the protective insulating film (passivation film) (contact hole in the protective insulating film), and a contact hole to be formed in a planarized film composed of an organic film and covering the protective insulating film (contact hole in the planarized film), are provided separately.
As a result, the contact hole in the planarized film is formed inwardly of the contact hole in the protective insulating film, and the transparent conductive film is formed on the side wall of the contact hole in the planarized film and on the electrode surface, as shown in FIG. 5.
According to a method of manufacturing the liquid crystal display panel disclosed in Japanese Patent No. 4487318, the contact hole in the planarized film is formed by dry etching. More specifically, a resist mask is formed, and selectivity between the resist and the planarized film (ratio between the respective etching amounts) is adjusted under conditions for the dry etching. This can form the contact hole in the planarized film into a tapered shape (shape where an opening part is larger in area than the bottom) relatively easily. This shape achieves favorable coating performance of the transparent conductive film. The aforementioned method can also form a contact hole in a planarized film made of a photosensitive material into the same shape.
In the liquid crystal display panel disclosed in Japanese Patent No. 4487318, however, a metal film as an electrode of the thin-film transistor and the planarized film directly contact each other, as shown for example in FIG. 10. The planarized film composed of an organic film and the like contains a slight amount of water and this water may generate corrosion of the metal film. Additionally, the planarized film on the metal film generally does not have strong adhesive force so it may become separated in some cases. Separation of the planarized film may generate a step cut of the transparent conductive film.
A material for the metal film and that for the planarized film should be limited in order to avoid the aforementioned problems. Thus, it is preferable that the protective insulating film cover the metal film from above to prevent contact between the metal film and the planarized film. To be specific, the protective insulating film is provided with the intention of shielding water and enhancing the adhesive force of the planarized film. Thus, a silicon nitride (SiN) film is suitable as the protective insulating film.
Preferably, a contact hole is formed in the planarized film on the protective insulating film in order to prevent contact between the metal film and the planarized film.
An SiN film is etched at a high etching rate. Hence, if the protective insulating film is composed of the SiN film for the aforementioned reason, the contact hole in the protective insulating film is formed into an upright shape and cannot be formed into a tapered shape easily, unlike the contact hole in the planarized film.
Additionally, a surface of the metal film at the bottom of the contact hole may be roughened to generate a gap (notch) in the outer circumference of the bottom of the contact hole. In this case, the coating performance of the transparent conductive film may be degraded at the gap to generate a step cut of the transparent conductive film, leading to the probability of display failure.